Feedback automatic volume control circuit for seismic amplifiers



y 1949. c. B. SCOTT FEEDBACK AUTOMATIC VOLUME CONTROL CIRCUIT FOR SEISMIC AMPLIFIERS Filed April 4, 1946 Output W QM ATTORNEY.

Input Patented July 5, 1949 FEEDBACKTAUTOMATIC VQLUME GONTROL" CIRCUIT .EOR 'SEISMIGZAMP-LIFIEBJS Clarence B. -Scott, Tulsa; 0kla.,..assignor.to Stand-Q.

ard Oil Development Company, a corporation. of Delaware Application Aipril 4,1946, Serial No.i659,4662' i The present invention is directed to a circuit..- for use in the recording of electrical signals in.' such a manner as to provide automatic-volume.

control.

In many arts there is encountered thenecessity..

for recording a varying electrical signal. When. this signal varies within wide limits, the difficulty encountered in the. recording is that if the recording apparatus is made sufiiciently sensitive to record the extremely low values, it usually is too sensitive to record the maximum 'values. .This is particularly true in the artof seismic .prQpeQt-v ing'where seismic waves arereceived,andconeverted into electrical valueswhich. arerecorded. on a moving strip of fixed dimenSiQnsMTheen-Q ergy received varies betweeniai'rlyfwide.limits and it is desired'to record all of it. Therefore,

the difficulty arises that, when the.recording apparatus is made sensitive. enough, to record low.

energy signals on the recording chartrit causes high energy signals to go nfi the chart. This same problem "is encountered in many types of analyses such as mass spectrographic analyses, infra-red ray analyses, and the like.

In such arts, therefore, theexpedien't ha'sbeen adopted of providing an auto-mati'c'control'o'n the sensitivity of the recorder or the amplifier which feeds the recorder, usually the latter, such that the greater the energy contained in the received signal the less will be the degree of amplification.

Theprincipal objective of the present invention is the provision of a simple circuit for achieving the aforesaid desired objective.

The nature of the present invention will be readily understood from the following detailed description of the accompanying drawings; in which,

Fig. 1 is a schematic diagram of one form of circuit which constitutes one embodiment of the present invention; and,

Fig. 2 is a similar View of an alternative embodiment of the present invention.

Referring to the drawing in detail, numeral I designates the terminals at which the varying signal is received. This signal is fed into a tube 2 which constitutes the first stage of amplification, the signal being applied to the grid 3 on which there is maintained a constant negative voltage from a source 4. In this particular tube the grid must be kept negative with respect to the cathode, the tube being a conventional pentode. As will be apparent, there is associated with this tube the usual circuit elements such as resistors, batteries, and condensers, all in the conventional manner. Since these constitute no 1 (Elaimi- (Cl. 179 171) part of -..the--present invention, specific. mention :7. thereof willbe omitted.

The-plate voltage of tube 2 is coupled to the grid 1 5 of a second pentode 6 which constitutes'rthe e second stageof amplification. It will be under: stood that as manystages of amplification as desired may be employed simply by multiplying the number of these tubes.-

Thaplateof tube 5 is connected to output .ter-: minalslto which is connected a recorder such as a recording galvanometer. .A portion5of the .plate current from tube 6" is drawn offzbywaynof cons ductor .8, whiohis connected to the-platen o'i control tube 16.: In:this tube a constanttnega- Y .tive. voltage is applied to the'grid l l. This negaa tive voltage is of such-aivalue as normally .to keep the-tubeextinguished; that'is, to keep the plate resistance extremely-high so that no. current will pass through the tube. The conductor .8 is also .coupledto the-grid I2 of an'amplifying'triode l3,

the:plate of which is coupled "to a rectifier i4, the .outputof which is fed'toithergridll of the control tube 10.

The rectifier output is a positive voltage which acts-in opposition to the fixed negative biasing voltage applied to the gridxll. Asthe positive voltageincreases, the'plate resistance decreases a and permits-current to ifiow'throughthe tube. This .c-urrentis taken 01? at thercathode I 5 of the" control. tube: and fed to the cathode "l6 of the I first. stage of amplification. :Since the-output of tube 6 is in phase With the input to tube 3, the change in voltage on the cathode [6 by reason of the output of tube [0 acts in opposition to changing voltage on the grid 3 of the pentode tube, thereby decreasing the degree of amplification attained in pentodes 2 and 6. It may be observed here that where more than two stages of amplifications are used, the feedback to the cathode of the first tube may require the use of a transformer in the feedback line to insure that the feedback voltage is in phase with or opposes the varying grid voltage on the initial stage of amplification. If an even number of interposed tubes is employed, the transformer is unnecessary.

In this embodiment, as the plate resistance of control tube I0 is reduced, some direct current plate voltage passes through the tube and to the cathode I6 of pentode 2. This causes surges which appear on the record as deviations of the base line from its zero value, giving erratic records. This can be eliminated if desired by interposing a high pass filter between the terminal 1 and the recorder, which will exclude from the recorder these lower frequency surges. In some cases, however, it is undesirable to use a filter at this point and for this reason the embodiment of the present invention shown in Fig. 2 is pre ferred.

In the embodiment shown in Fig. 2, the circuit is the same up to the control tube it). In this case conductor 8 is connected to the grid ll of a phase changing tube H? which puts out a plate signal and a cathode signal which are 180 out of phase with each other. The plate signal provides voltage for the grid E9 of one control tube 28, while the cathode signal provides voltage for the grid of a second control tube 22.

As in the case of Fig. l, the output of the rectiher It is a positive voltage which opposes the fixed negative cutofi bias applied to grids l9 and 2| of control tubes and 22 respectively. The plates 23 and 2 of tubes 28 and 22 respectively are connecwd to the end terminals of a transformer 25 which has a central tap 2t connected to a B battery to provide plate voltage for the tubes 20 and 22. The secondary 2'! of this transformer is connected to ground. and to the cathode I6 of pentode 2.

In this case, the surge effect is eliminated since it is balanced out b the action transformer" and the tubes 2!] and 22 as is clearly shown the following explanation: Variations in bias voltage from rectifier M which produced surges in the circuit in Fig. l are applied in Fig. 2 to l5! and 2'! of tubes 28 and 22 respectively in equal amounts and in phase. These grid bias variations produce plate current variations at plates 23 and 24 which are also equal an in phase. Since equal and in phase variations of current at end terminals 23 and 2d of the transformer winding for which terminal 25 is a center tap roduc equal and opposing transformer 25, no voltage is developed across secondary ing El of the transformer and no surge is applied to the amplifier stages formed by tubes 2 and E.

The amplified signal voltage apur earin on condoctor 8 is applied to grids id ll of tubes 29 and 22 180 out of phase by virtue of changing tube 48 as previously explained. The signal voltage is therefore transmitted through control tubes 28 and 22 and transformer 25 and applied to the cathode is of tube 2 in such a manner as to oppose the input signal applied to terminals 5 of tube 2. The net gain of tubes 2 and 6 is thus automatically controlled by control tubes 2b and otherwise noted.

The nature and objects of the present invention having been thus described and illustrated, what is claimed as new and useful and as desired to be secured by Letters Patent is:

An amplifying circuit for use in amplifying a varying electrical signal for recording, comprising a plurality of amplifier tubes, means for applying the varying signal to the grid of the first amplifier tube, means for taking oil the amplified signal from the plate of the last amplifier tube, a feedback arrangement between the plate of the last amplifier tube and the cathode in the first amplifier tube, includin a phase changing tube having its grid connected to the plate of the last amplifier tube and being of a character to produce a plate signal and a cathode signal out of phase with each other, a pair of control tubes, one of which has its grid connected to the plate of said phase shifting tube and the other of which has its grid connected to the cathode of said phase shifting tube and both having their plates connected to opposite ends of the primary of a transformer, means for applying a rectified portion of the signal derived from the plate of the last amplifier tube in equal amounts and in phase to the grids of said control tubes, a central tap on said primary winding for applying plate voltage to said control tubes and means connecting the secondary of said transformer to the cathode of said first amplifier tube.

CLARENCE B. SCOTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,307,308 Sorensen Jan. 5, 1943 2,369,066 Maxwell Feb. 6, 1945 

